When a particularly finely divided, especially jet-ground material is processed in classifiers based on centrifugal force, it is necessary to aim at a very high inlet velocity as well as at such a gas-solids suspension in which the excess quantity of gas is very large. When the difference in size between the solid particles to be classified is reduced, the difficulties in obtaining a satisfactory result of classification are increased very steeply. This comes from the fact that, when the particle size is very little, for example 1 .mu.m and less, the differences in conduct obtainable by means of centrifugal force between the particles of different sizes are extremely little, which imposes very high requirements on the classifier
In the prior-art embodiments wherein the gas-solids suspension rushing out of the jet grinder is passed directly into the classification chamber, the solids content in the gas-solids suspension is relatively high. In order that a good grinding capacity and economy could be obtained, it is, namely, required that the solids contents in the gas-solids jets rushing into the grinding chamber are kept at a relatively high level, in order that the probability of collision of the solid particles should be sufficiently high and that the consumption of "expensive" high-pressure air should remain within reasonable limits. In order that a good result of classification could be obtained, therefore, attempts have been made to introduce additional air into the classification chamber, e.g., through tangentially directed additional-air nozzles. In practice, it has, however, been noticed that these additional-air jets cause flow phenomena that disturb the process of classification, so that, with the prior-art equipments it has proved extremely difficult to obtain a satisfactory result of classification in respect of ultrafine material.
The difficulties in classification of ultra-fine solid material come out clearly-from an experiment of classification and grinding, which has been carried out in practice, which is examined from the point of view of calculation, and wherein it has been studied how the velocities of particles of different sizes (density =2750 g/cm.sup.3) are changed as a function of the distance of the particle concerned after the acceleration nozzle that accelerates the gas-solids suspension. The following table gives the theoretical values for the deceleration of particles of different sizes after the nozzle from the initial velocity v.sub.po as the distance becomes longer. The table also clearly indicates the significance of the feed-in velocity of the particles for classification and grinding.
______________________________________ Particle Theoretical deceleration at a distance of size 1 cm 3 cm 5 cm 10 cm .mu.m m/s m/s m/s m/s ______________________________________ 1 decelerated immediately to the velocity of the gas effective in the space 5 60 180 decelerated to the velocity of the gas effective in the space 10 15 45 75 150 20 5 10 20 40 50 1 2 3 6 ______________________________________
From the table it comes out that particles of a size of 1 and 5 .mu.m are almost immediately adapted to the velocity of the gas effective in the space, so that separation of particles of 5 .mu.m from a gas-solids suspension is very difficult and requires a classification chamber of relatively small diameter.